Electric heating unit



April 14, 1953 N. G. REHNQVIST ET AL 2,535,164

ELECTRIC HEATING UNIT Filed Aug. 21, 1951 2 SHEETS--SHEET 1 6&1: fa E ri/f H i/debraha f/l/debrand Affarngy April 14, 1953 Filed Aug. 21, 1951 N. .G. REHNQVIST ET AL ELECTRIC HEATING UNIT 2 SHEETSSHEET 2 Afforn 6y Patented Apr. 14, 1953 v...

ELECTRIC HEATING UNIT Nils Giista Rehnqvist and Giista Erik Hildebrand Hildebrand, Hallstahammar, Sweden, assignors to Aktiebolaget Kanthal, Hallstahammar, Sweden, a corporation of Sweden Application August 21, 1951, Serial No. 242,808

4 Claims. (01. 201-457) This invention relates to a new and improved electric heating unit of the type comprising at least one metallic resistance heating element embedded in an electrically insulating heat-conducting mass such, for example, as magnesium oxide, which in turn is encased by a metallic case.

The object of the invention is to produce the heating element as well as said case from alloy metals which besides possessing satisfactory physical qualities will minimize deterioration of the insulating mass at elevated temperatures over extended periods of use.

According to the invention the alloy metals of said heating element and case have iron bases and also contain aluminium and chromium. Small amounts of other elements may be present in the alloys as impurities or may be added during the smelting process in order to improve heat resistivity and mechanical properties of the alloy metals.

As explained in a still pending application numbered 191,693 and filed on October 23, 1950, the deterioration of the insulating mass may be substantially dependent on a diffusion of chromium oxides and other oxides into said mass. Especially chromium oxides possess considerable electrical conductivity and consequently diminish the insulating capacity. In order to avoid this the inventor of the above application proposes that the case be made of an alloy containing aluminium, the base metal being iron or nickel. Aluminium oxide has a very low electrical conductivity and forms a protecting layer on the inner surface of the case, so that chromium oxide or other oxides are prevented from penetrating the insulating mass. The reason for his proposal may be that the inner surface of the case, which is relatively large compared with the surface of the filament, is in the first hand responsible for the diffusion of chromium oxide into the insulating mass. However, we have now found that in spite of its minor surface the filament has at least as great an influence as the case upon the insulating mass. This is illustrated on the accompanying drawings, in which Figs. 1 and 2 show photos of sections of heating elements in an enlarged scale, thefilament as well as the case consisting of nickel-chromium -alloys. Fig. 1

dark-coloured around the filament owing to oxides oi. nickel and especially chromium. In Fig. 2 the dark-coloured range has been extended so that it contacts the inner wall of the case. Thus, it is evident that the deterioration of the insulating mass is to a large extent caused by the filament, which should mainly be due to the fact that the filament is heated to a considerably higher temperature than the case, and consequently causes a more rapid diiiusion of oxides into the surrounding mass. Thus, according to the invention, a content of aluminium in the case as well as in the filament will highly improve the heating unit.

Other objects and features of our invention will be apparent from the following detailed description. An embodiment of the electric heating unit referred to is shown in Fig. 3 on the drawings, partly in section. Said unit is of the type having a spiral form, but may, of course, have any convenient form without effecting the invention,

such as straight, rectangular, circular etc.

In Fig. 3 the numeral 4 indicates a metallic resistance heating element formed as a helical filament. 5 is an electrically insulating heat-conducting mass. 6 is a metallic case.

The case may be made from a metal strip which is bent in the longitudinal direction to the form of a tube, whereupon the longitudinal edges of the strip are welded together. When the filament and the surrounding insulating mass are placed in the tube the ends of the latter are sealed in any suitable manner.

As mentioned above the alloy metal used for the case contains iron, chromium and aluminium. Preferably we use an alloy containing 5-30% chromium, 2-9% aluminium, the balance principally iron. To improve the heat resistivity the case alloy may furthermore contain other constituents such as zirconium, calcium, thorium, cerium and others, and to increase the ductability the alloy metal may also contain titanium, niobium, tantalum and thorium.

For instance, it may be mentioned that alloy metals for the case have been found to be satisfactory with respect to insulating properties when composed of the following indicated proportions of constituent elements, expressed in percentshows how the embedding mass begins to become ages:-

I Table 1 O I Si Mn Cs Ce Zr Tl Nb Ta 00 Fe 0.00 0.0 0.0 0.0 0.01 0.0 0.0 0.0 0.0 1.0 Balance. 0.10 0.0 0.5 0.0 0.05 0.1 1.0 3.0 0.0 10.0 Do. 0.20 0.1 0.5 0.0 0.4 0.15 0.0 1.4 3.0 0.1 D5. 009 1.2 0.0 0.4 0.0 0.0 0.0 0.0 0.1 0.0 Do.

The resistance heating element or elements, 1. e. the filament, is also formed from an alloy containing iron, chromium and aluminium. Of course, it is here the question of a high quality heat refractory alloy. Preferably, we use an alloy containing 15-25% chromium, 3-6% aluminium and -3% cobalt, the balance principally iron. In the same way as the alloy used for the case the filament alloy may contain constituents such as zirconium, calcium, thorium, cerium, titanium, niobium, tantalum to improve the properties. The following table shows some preferred compositions, the values being indicated in percent.

Table 2 Al Cr C Si 00 Ni 02 Other metals Fe 22 0.05 1 v 0. 45cc Balance 10 0. 35 1 0. SN?) 1. lli Do. 30 D. 0. 5 0, 250a+2. 4Ta Do. 0. 9D 1. 5 3. 75V D0. 28 0. l5 2. 5 0. 43Ba+2 4Ta Do. 30 0. l5 2. O O. Ca+0. 65V Do. 20 0.23 l. 5 3. 5V D0. 40 0. 50 2. 5 3. 8Z1 Do. 20 0. l. 4 2. 25Nb+0. 4T1 Do.

' It has further been found that the abovementioned destruction of the heating element may occur after a long period of use even if the filament and the case are of iron-chromiumaluminiuin alloys, as proposed by this invention. The reason for this is that the protecting layer of aluminium oxide cannot always be regenerated to the same extent as it scales on". This is in turn due to the fact that a sufficient amount of oxygen is not always present, in that the initial amount of oxygen is consumed and the structure of the element not permitting supply of further quantities of oxygen. Thus, in this case the atmosphere in the heating unit will consist substantially of nitrogen which reacts with and successively damages the resistance material. Consequently, it is of importance that a protecting layer of aluminium oxide always is present and successively regenerated during the whole life of the unit, and the invention also includes means for securing the regeneration of the aluminium oxide layer.

For that purpose the heating unit may at its manufacture for instance be filled with oxygen up to a suitable pressure and then sealed. In this case the air in the unit is first removed by blowing with oxygen or by exhaustion, so that substantially all nitrogen is away. Then the unit is filled with oxygen the pressure being adjusted in dependence of the strength of the case and the operating temperature adapted for the element. According to another proposal the insulating mass may from the beginning be admixed with any substance delivering oxygen at the operation temperature of the tube element, such as M1182, BaOz or Caoz, preferably MnOz. case must be closed to prevent escape. of oxygen. 'The quantity of such oxygen delivering sub stance is determined in the first hand by the amount of oxygen that said substance is capable to liberate at the actual temperature. In using MnOz, for instance, the quantity oxygen delivering substance may amount to about 0.5%. Also in this case the unit must in some waybe freed from nitrogen before it is closed.

Of course, the

ducing mass, said mass being contained in a metallic case, said heating element and said case being formed from metal alloys in which the base metal-is iron and the alloying metals include aluminum and chromium, and said electricallyinsulating, heat-conducting mass being mixed with an oxygen-releasing substance.

3. An electric heating unit comprising at least one metallic resistance heating element embedded in an electrically-insulating, heat-conducting mass comprising magnesium oxide, said mass being contained in a metallic case, said heating element and said case being formed from metal alloys in which the base metal is iron and the alloying metals include aluminum and chromium, and said case containing oxygen under pressure.

1. An electric heating unit comprising at least one metallic resistance heating element embedded in an electrically-insulating, heat-conducting mass comprising magnesium oxide, said mass being contained in a metallic case, said heating element and said case being formed from metal alloys in which the base metal is iron and the alloying metals include aluminum and chromium, and said magnesium oxide insulating mass being mixed with an oxygen-releasing substance selected from the group consisting of MnOa, 138.02, and CaOz.

NILS Gos'rA REHNQVIST.

Gos'rA ERIK HILDEBRAND OTHER REFERENCES Woldman, Materials Engineering of Metal Products, 1949, chapter XIII, pp. 335-349. 

